Torrefaction: Mechanistic study of constituent transformations in herbaceous biomass

• Hydrolysis coupled to HPLC analysis complements TGA in constituent characterization.
• Structural carbohydrates in bagasse are stabler than those in verge grass.
• Extractives may significantly affect energy and mass yields in torrefaction.
• Carbohydrate degradation in verge grass leads to more ‘char’ formation.
• Energy yields are more strongly affected by mass loss in bagasse torrefaction.

Several improvements in biomass properties due to torrefaction can be attributed to a chemical transformation of its constituents. This study focusses on investigating the nature of these transformations along with their association with mass and energy yields for varying conditions of torrefaction temperatures and times for verge grass and bagasse. Torrefaction was carried out in a 0.87 l bench-top fixed bed reactor convectively heated with a nitrogen flow. Chemical analysis of the products was carried out using high performance liquid chromatography (HPLC) following a two-step acid hydrolysis, whereas the applied thermal analysis techniques included differential thermogravimetry and bomb calorimetry. It was seen that for a torrefaction temperature of 290 °C at a residence time of 15 min, approximately 23% glucan and 82% xylan in bagasse is converted as compared to 96% glucan and 97% xylan that is converted in verge grass. In addition to the degradation of structural carbohydrates, the extent of formation of acid insoluble residue (char) was also seen to differ with the choice of feedstock. As a consequence, within the range of torrefaction conditions, a comparable reduction in mass yield is found to lead to a drop in energy yield that is 25% less in the case of verge grass as compared to bagasse.